Alternating current signaling system



ALTERNATING CURRENT SIGNALING SYSTEM Filed Aug. 25, 1932 2 Sheets-Sheet l glw g R I i I Q: P

c, T [firm rot- Dec. 26, 1933. E RYALL 1,941,229

ALTERNATING CURRENT SIGNALING SYSTEM Filed Aug. 25, 1932 2 Sheets-Sheet 2 [ma/won- Zzomm [P/VEJT F /uz fiatented Dec. 26,

@ UNITED STATES PATENT OFFICE i V w Q 1,9 41,229 g I I ALTERNATING CURRENT SIGNALING SYSTEM. o Leonard Ernest RyaIL' 'Sutton EngIand, assignor, p to Associated Electric Laboratoriesglna, Chicago, 111., a corporation of Delaware I Application August 25, 1932, Serial No. 630,399,

and. in

' The present invention relates to alternating current telegraph systems and is more particular- 1y concerned with systems of-the type in which the so-called carrier alternating current' isfof a 5 frequency which ishigh comp'ared withthe'frequency of interruption constituting the signals; It may readily be proved that when the 'flowof current of carrier frequency is started orstopped, currents are generated having a frequ ency range which is theoretically infinite. If the circuit in question is arrangecl'for single channel working only so that all these currents'can be transmitted, no difficulties are encountered and the signals arrive at the receivingend without sensible distortion. If, however, as will usually be the case, the circuit is arranged for multi-channel working with carrier currents of different frequencies, it is important that the currents generated when current of any particular carrier frequency is started or stopped shall'be confined to a given frequency range soas not to interfere withsignals sent on other carrier frequencies. The ideal to be aimed at inworking systems of this type is that the envelope of the signal cur 25 rents shall be rectangular and this will ordinarily only be. attained if an infinite frequency range is possible. Where, as: for commercial multi-channel working, only a limited widthfof side band is available, for instance 60 cycles on 30 each side of the carrier'frequency, distortion is introduced and the envelope assumes a more nearly sinusoidal shape. This has the undesirable effect of making the signals more difficult to receive satisfactorily so that forthe'same factor of safety the speed of transmission is reduced; The chief object of the invention: is to provide anai' rangernent whereby in a system of the type described this distortion is counteracted and the signals are'restored to something approaching 40 their original rectangular form and are independent of the magnitude of the incoming signal voltage over wide limits and moreover are independent of the presence of signalstransmitted over adjacent frequency channels. Tl'liSLIBSlllt'jlS achieved by the use of apparatus of a very simple and inexpensive character' ff It will be appreciated from what has'beensaid above that fer'the 'operation of a multi channel system some kind of filter or tuning arrangement is essential even if the carrier frequency as originally generated is kept accurately 'tojits nominal value. Consideration will show also that the tuning device is preferably not such as to 'be sharply resonant but to give a response which is 55, substantially the same for the whole width of side Great Britain-August 25, 1931 14 Claims. (01. 178 -51) band concerned. 7 In other words response plotted against frequencyshouldgive a curve in'the gem eral form of a truncated cone with a substantially 'fiat'top; In arrangements previously used-a band pass-filter closed with its characteristic impedd ance has generally been employed for restricting the response of theequipment to the particular frequencies concerned, and-giving verylarge at tenuation to signals of frequencies outside this frequency band, but this is always an expensive piece of 'equipment'and according to the invention satisfactory results are obtained by the use of simple tuned'circuits arranged to, have a uniform response over the band required, but in which the attenuation may not rise to such a high value for frequencies considerably outside this band as in the case of an ordinary band pass filter. It is arranged that the tuning circuit has two slightly differentresonantfrequencies which are preferably equidistant from'the carrier fre" 7 quency, so that the response curve of such an arrangement will have two humps symmetrically located with respect to the carrier. frequency which will correspond to the depression between.

This tuning'arrangement is employed at both the transmitting and receiving ends of the :cir-

cuit but the frequencies to which the filtering arrangements are tunedat thereceiving end are closer together so "that the two humps in there-- sponse curve overlap with the result that the combination of the two tuning circuitsproduces a peak which 'is 'substantially'flat over thewhole width of the'side bands concerned I According to one feature of'the inventiomin an alternating current signaling system employing 'a'carrie'r current of a particular frequency'the input from the line is'passed' through a simple resonant circuit or circuits responsive to. the carrier frequency, is limited by means of a gas discharge tube and'is then fed byway of a resistance and afurther tuned circuit-and a rectifier to a thermionic valve having a responding relay in the'anode circuit.

--According to another feature of the invention, signalstransmitted'with one carrier frequency m produce corresponding: reductions in the cathodeano'de impedance of-a' thermionic valve this'circuit including an alternating current source'pf another 'carrier' frequency and a transformer tuned by means of abondenser to this frequency a I so that Whenthe impedance of the valve is-reduced in response to a signal an alternating volta'ge'of the secondifrequency will be impressed on the :secondary of; the transformer. which is connected toa furthersignalingline,

the battery. The contacts of the relay are shown connected to give double .current working in the output circuit over leads 46 and 47 but obviouslyany suitable connections may be made. The valves used are of the'indirectly heated type and the heaters are shown connected in se"-- ries to be supplied frombattery 48 which may have agvoltage of 2.4. A resistance 43may be used tojbring the current to the desired value or the heaters of corresponding valves in' the receiving rier frequency are passed to the tuned circuits at the transmitting end and are amplified-by the amplifierl2 and-transmitted over the line. In a similar manner currents of the othercarrier.

frequencies concerned may be flowing overthe line from their respective transmitters. Furthermore, as previously pointed out, when the carrier currents are started or interrupted, currents ofa large number of unwanted frequencies are generated which are not completely suppressed by the somewhat coarse filtering circuits used.

.At the receiving end the filtering arrangements though of a simple nature are nevertheless sufil-.

ciently selective to ensure that the signals passed .to the receiving equipment for-any one-chan-- nel shall contain a very large proportion of, signals of the correct frequency and consequently the power of the correctfrequency signals rep-.

resents practically the whole of the total power. As previously mentioned the grid potential of the amplifying valve 23 is controlled from the anode circuit by the drop in resistance 30., "The use of the-condenser-resistance combination 26 and 27 in conjunction with the rectifier 28 ensures that alternating current signals in the anode circuit will charge up the condenser 26 and thus increase the negative grid bias. The potentiometer29 enables a positive potential to bev obtained tocounteract the large grid voltage v which would otherwise be produced when no signal was present. The values of condenser 26 and resistance 27 are so chosen that the discharge time of the condenser is longer than the interval between successive signal pulses and? consequently when a current of large voltage amplitude is received, a considerable, negative grid bias is generatedwhich persists during the time that a small voltage is applied corresponding to the interval between successive signal pulses. The eiiect of this arrangement is'that due to the grid bias the small signal is not .amplifled to any extent and thus the separation between successive signal pulses is accentuated. This serves to compensaterfor the effect of the limiting arrangement in the succeeding stage, the.

tendency of which particularly in the case of largep signals having a substantially sinusoidal envelope, is to increase the signal period and reduce the, spacinginterval. The resistance 24,

onsidering now the operation of the circuit, whenthe key 2 corresponding tov the channel. shown isclosed, currents of the appropriate .car-.

rent any change of potential which might be produced'by the flow would be effected slowly relatively to the signal pulses and thus the conditions of operation would be rendered different according as a short or long-signal'had been received. In order to producethis result satisfactorily resistance 24 mustbe of a high value compared with resistance2'7 and 5' megohms has been found suitable in'practice. Ill order, however,to avoid prejudicing the amplifying properties of the valve'by the introduction ofa high impedance into the grid circuit, condenser 25 is shunted across'resist'ance 24 to provide a low impedancepath for the alternating currents. I Itwill be appreciated, however, that the resistance-condenser combination24 and25 is not required to increase thetime during which the negative grid potential due to received signals persists,- and consequently the discharge time of the combination 24 and 25must be sufiicientlyshort that it can follow the signal envelope. For this purpose it is suitable'for the condenser 25 to have a value of .001 mi. if resistance 25 is 5 megohms as suggested and the maximum signal impulse frequency is about-30 cycles per second. In these circumstancescondenser 26should have a value of at least 4 mt. and resistance 2'7 which is small compared with resistance 24 may be megohm.

As regards the value of resistance 30, it is necessary thatthe power in this resistanceshall be signal envelope attains a maximum value for-- only a very short period during each impulse.

The .value of resistance 45 determines the amplitude of the signal voltages received acr'oss' the'windings of transformer 34. i If the value of resistance-.45 is increased, the frequency-band within which the voltage across transformer 34 is sufiicient' to cause anode' current tofiow in valve .40 is reduced and thus this resistance can be usedto control the width" ofthe frequency band within-which the signalsr'nust lie in orderto cause the operation of relay 41; As the be- 7 ginning and end 013a signal impulse'consist ofside band frequencies: removed from the carrier frequency, it follows that alteration" of resistance 45"may be used t'o-var y the time-during which relay41 remains operated in'rsponse to' an impulse signal of given length. I It is an important feature of cuit that thenormal negative grid-cathode bias of the rectifying valve 40 is'very considerably rent from flowing Hence noanode currentis produced until a voltage above apredetermined callydependent on" the appliedvoltage to the grid, itis important that the incoming signal voltage tothis part of the receiving system shall be controlled to be constant within very narrow the receiving cirgreaterthan that required to prevent anode cur- H limits in spite of variations inthe initial signal preferably aneonlamp, which acts to limit the a received signal to a constant amplitude. The;

voltage applied to the grid is then determined.

entirelyby the-frequency of the signal in relation to the frequency-impedance characteristics pulse.

of the tuned circuit to which it is applied. More specifically, if a signal consists almost entirely of the carrier frequency, the tuned circuit 33, 34 will be in resonance and will have its maximum impedance and consequently the voltage passed to the secondary of transformer 34 will be a maximum. As the signal departs from the resonant frequency, the impedance of the tuned circuit will decrease and consequently more of the applied voltage will be absorbed by resistance 45 so that a point is soon reached at which the applied voltage is insuflicient to cause a flow of anode current sufficient to operate'relay 4-1.

It will be appreciated that for the limiting action of the neon lamp to produce the desired effect it is important to employ some filtering arrangement in the preceding stage to ensure that the voltage of signals of the required frequency is always practically equal to the total voltage of the limited signals so that its magnitude is independent of the presence of currents of other frequencies. This permits the use of a simple filter or tuned circuit in the succeeding stage to prevent currents of other frequencies from causing operation of the responding relay.

As regards the resistance-condenser combination 36 and 3'7, this operates in conjunction with the rectifier 35 to produce a steady voltage on the grid and thus give rise to a substantial isteady current in the anode circuit. It is clear that the condenser must not retain its charge between signal pulses while at the same time it should not lose it appreciably between individual pulses of carrier frequency constituting a signal The values of the condenser and resistance must be chosen with this in mind and also the desideratum that the signal distortion occurring due to the time taken to charge the condenser should be compensated by the-time taken to discharge the condenser through resistance'36. The resistance 38' is employed so that the potential of the condenser 3'? may riseto a value somewhat higher than would ordinarily be the case if the limit set by the flow of grid current were applicable. Accordingly condenser 3'7 may discharge appreciably between carrier frequency pulses without the voltage across its terminals dropping to the value-to which it would have been limited by the flow of grid current in the absenceof resistance 38.

,It should be mentioned that the tap of inductance 6 which is included in the second resonant circuit should be small soas not to disturb the properties of the first resonant circuit, and for a similar reason'it. is important that the resistance 9 shouldbej of comparatively small value. It will be appreciated that" the samereascningapplies to the tap-of the inductance l7 and the resistance 21 located-atthe receiving end,

' these circuits being of the samedesigrras those at the transmitting end.

It may happen incertainjcircumstan'ces. that it is desirable to convert signalsfof one carrier" frequency intosignals of adifferent carrier frequency; such a necessity; might arise' for i'n'-- stance in case multi-channel,working-took place overan important trunk toa distributing point and the signals were then transmitted to the various receiving points by means of single channel working. This conversion can readily be effected without the-use of any moving partsby, aslight modification of the arrangement" shown in Fig. 1 which is illustrated in: Fig. 2.'' The complete circuit at the receiving station has 50 occupiesthe' same position as the valve 40 in Fig. 1. Instead of the responding relay the anode circuit of the valve 50 includes the secondary ofa transformer 51 fed from the source 52 which supplies alternating current of the carrier frequency to which it is desired to convert. The circuit also includes the primary of' a transformer-53 which in conjunction with the condenser 54 is tuned to have a high impedance to currents of other frequencies including the original carrier frequency. When signals of the original frequency are received causing the anode current of the valve 50 to increase, the anodecathode' impedance of the valve decreases from practically an infinite value to a comparatively low value and accordingly an alternating voltage of the required carrier frequency is obtained across the secondary winding of the transformer 53 and thus is fed to the outgoing line 55. Consequently each time currents of the original carrier frequency are received corresponding currents of the new carrier frequency will be transmitted over the outgoing line.

What I claim is: i

1'. In a signaling system for transmitting signals of interrupted carrier current over a line,

a "gas discharge tube for limiting the input from said line, simple resonant circuits included in said line ahead of said discharge tube, a thermionic relay having a responding relay in its anode circuit, and circuits including a resistance, a tuned circuit, and a rectifier for feeding the limited input controlled by said discharge tube tosaid thermionic valve.

2. In a signaling system for transmitting signals of interrupted carrier current over a line, a limiting device included in the receiving end of said line, tuned circuits included in said line ahead of said limiting device, a relay, and a simple filter circuit interposed between said limiting device and said relay for controlling the operation of said relay only in responseto signals of said carrier frequency of the proper magnitude. 3. In a signaling system for transmitting sig-' nals ofinterrupted carrier frequency having a narrowside band, a thermionic valve, a limiting device for controlling the output of said valve, said limiting device having a tendency to lengthen thesignal impulses, and means including circuits for modifying the amplification characteristic of said valve by increasing the negative grid voltage in response to the receipt of signals of large voltage amplitude thereby shortening the signal impulses received in order to compensatefor the action of said limiting device.

4. In a signaling system for transmitting signals of interrupted carrier frequency having a narrow side band, a thermionic valve, a limiting device for controlling the output of said valve, said limiting device having a tendency to change the" length of the signal impulses received, and means including circuits for altering the grid bias of said valve in response to the receipt of signals of large voltage amplitude thereby altering the signal impulses received in order to compensate for the action of said limiting device.

5. In a signaling system for transmitting signals of interrupted carrier current over a line, a thermionic valve having a grid'and an anode circuit at the receiving end of said line, a resistance and a condenser combination, the input at the receiving end of said line being fed to' the grid of said valve by way of said resistance and condenser combination, a rectifier for recti not been show n but it is assumed that the valve fying the outputiof said valve, said condenser nals of interrupted carrier current of a partic-.

' gas discharge-tube for limiting'the inputfrom said line, simple resonant circuits included in the effector entirely preventing-the passage of signals of small magnitude between signals of large magnitude.

6. In a signaling system, a first alternating current source of a particular frequency, a thermionic valve, a second alternating current source of a different frequency, a signaling line, a tuned circuit for said second frequency including a transformer and a condenser connected to said signaling line, said transformer coupling said tuned circuit with the anode circuit of said valve, said tuned circuit having a high impedance to currents of frequencies other than said second frequency, means for transmitting a signal of the first frequency to said valve, the anode current of said valve increasing responsive thereto for reducing the anode-cathode impedance of said valve, and a second transformer included in the anode circuit of said valve and connected to said second source for, impressing a signal of the second frequency on said signaling line in response to said reduced impedance.

7. Ina signaling system for transmitting sigular frequency over a line, the transmitting circuits of said line including resonant circuitstuned to two frequencies respectively slightly above and slightly'below said carrier frequency, the receiving circuits of said line including resonant circuits tuned to two different frequencies also respectively slightly above and below said carrier frequency, and means including said circuits in tandem for producing substantially flat response curves over the range representing the width of the side bands employed'in response to the transmission of signals of said particular frequency over said line.

8. In a signaling system as claimed'in claim '7 wherein the two tuned resonant circuits in the transmitting and receiving circuitsconsist substantially wholly ofcapacity and inductance, and in which the second of each of the two tuned circuits includes only a small tapping of the inductance of its associated first circuit. 9. In a signaling system as claimed in .claim '7 including alow resistance connected in series with the inductance and capacity of the second tuned circuit'in the receiving circuit for controlling the output fromsaid line.

10. In a signaling system for transmitting signals of interrupted carrier current over a line, a

the negative grid bias on said valve;

said line ahead of said discharge tube, a thermionic valve having a responding relay in its anode circuit, circuits including a resistance, a tuned circuit, and a rectifier for feeding the limited input controlled by said discharge tube to said valve, said valve havinga normally high grid bias for restricting the-flow of anode current through said relay until the limited input current reaches a predetermined point.

11. In a signaling system for transmitting signals of interrupted carrier current over a line,

- a gas discharge tube for limiting the input from said line, a thermionic valve having a responding relay in its anode circuit, a tuned circuit for feeding the limited input controlled by said discharge tube to the grid circuit of said valve, a rectifier, a condenser and a resistance in said grid circuit, the input from said tuned circuit being appliedthrough said rectifier to said condenser and resistance in parallel to the grid of said valve, said valve having a normally high grid biasfor restricting the fiow of anode current through said relay until the said input current reaches a predetermined value, and a further resistance connecting the grid-of said'valve to said rectifier for maintaining the voltageacross said condenser above the fiow of normal grid current during signal transmission.

12. In a signaling system as claimed in claim 1 including a resistance in said tuned circuit, the value of said resistance in the tuned circuit determining the length of the signal fiow in the device for controlling the output of said valve,

saidlimiting device having a tendency to change the length of the signal impulses received, and circuits including a resistance in the cathodeanode circuit of said valve, a rectifier, a condenser and .a resistance in parallel in the grid circuit of said valve for altering the grid bias of said valve in response to the receipt of signals of large voltage amplitude thereby altering the signal impulses received in order to compensate for the action of said limiting device.

14. In a signaling system for transmitting signals of interrupted carrier current over a line, a neon tube for limiting the input from said line, simple resonant circuits included in said line ahead of said neon tube, a'first thermionic valve, a second thermionic valve, a transformer having three windings, the first of said windings connected to said neon tube, the second of said wind ings included in the anode circuit of said first valve, and the third winding for feeding the signaling currents to said second valve.

LEONARD ERNEST RYALL. 

